<?php
/**
 * Created by XjHome.
 * User: xj
 * Date: 2015/5/21
 * Time: 0:05
 * 
 */
abstract class a
{
    abstract public function install();
}

class b extends a
{
    public function install(){

       $install=new b1();
        return $install->install();
    }

}

class b1 extends a{

    public function install(){

            echo '这里是B1的install';
    }
}

    $b=new b();
    $b->install();
